The long-term goal of this project is to determine the molecular basis of the inhibitory effects of cytochalasins (a group of fungal metabolites) on a wide spectrum of cytoskeletal and contractile functions in animal cells and tissues. The results to be obtained from this study will enable us to use these drugs effectively as diagnostic tools in future biomedical research. Moreover, this study is also aimed at yielding information on the molecular mechanisms by which a cell controls its motile functions. It has previously been shown that cytochalasins have the ability to interact with actin, a major protein component of the cytoskeleton of nonmuscle cells. In the proposed research, we will continue our use of a combination of biochemical, biophysical, and cell biological techniques to probe at the action of cytochalasins and related protein factors on actin structure and function in the following ways. (A) The high-affinity and low-affinity binding of different cytochalasins to monomeric and polymeric actin will be measured with standard pharmacological techniques. The binding data will then be correlated with studies on the effects of the drugs on actin filament assembly, disassembly, and network formation in vitro, under defined conditions and using purified protein components. (B) The efects of cytochalasins on actin-containing structures in cellylar and subcellular systems will be determined. This information will be related to the concentration dependency of the inhibition of a number of motile functions by cytochalasins, and to the data obtained in the in vitro experiments described above. (C) The structure of actin-containing cytochalasin binding complexes isolated from motile cells (e.g. human platelets) will be studied. Experiments will also be conducted to investigate the mechanism by which these complexes associate with cellular membranes. (D) Comparative studies on a class of proteins with cytochalasin-like activity against actin filament assembly and interactions will be continued. Special emphasis will be directed at the possibility that these proteins may function as regulators of actin filament assembly and membrane attachment in the cell.